1. Field of the Invention
The present invention relates to a vacuum type booster and more particularly to a booster of the type including a booster shell, a booster piston accommodated in the interior of the booster shell in a manner reciprocatively movable forwardly and rearwardly and dividing the interior of the shell into a first working chamber leading to a vacuum source and a second working chamber adapted to be selectively communicated with the first working chamber or the atmosphere via a control valve, a valve sleeve for the control valve which is fitted into a valve sleeve fitting hole located in the central part of the booster piston and which has a disengagement preventive flange superposed on the fore surface of the booster piston, and a clamp plate placed in resilient engagement with a plurality of engagement pawls on the booster piston for clamping the disengagement preventive flange in cooperation with the booster piston.
2. Description of the Prior Art
This kind of vacuum type booster is provided with a plurality of engagement pawls on a booster piston. The pawls are formed by cutting and bending operations so that the booster piston becomes weakened due to provision of holes which are formed as a result of cutting and bending operations for the engagement pawls. In view of possible weakening of the booster piston as mentioned above, a conventional booster is formed with reinforcement ribs between the aforesaid holes and a valve sleeve fitting hole in order to inhibit the booster piston from being weakened. However, the aforesaid reinforcement ribs could serve to reinforce only the surrounding area of the holes resulting from cutting out the pawls.
Further, a conventional booster includes a plurality of engagement pieces formed on the clamp plate so as to project radially outwardly of the disengagement preventive flange and adapted to come in resilient engagement with a plurality of engagement pawls on the booster piston. However, due to the fact that the outer peripheral part of the fore surface of the disengagement preventive flange is located very close to the engagement piece joining portions of the clamp plate, a fulcrum for allowing flexing of each the engagement pieces is located on the outer peripheral edge of the flange fore surface. As a result, the distance between the engagement piece and the fulcrum is quite short and a satisfactory spring function can not be obtained for the engagement pieces, making it difficult to achieve engagement of the booster piston with the clamp plate.
Further, a conventional booster is so constructed that a diaphragm, having an outer peripheral bead immovably secured to the peripheral wall of the booster shell and an inner peripheral bead secured to the opening edge of a valve sleeve fitting hole located at the center of the booster shell, is superposed on the rear surface of the booster piston. When the clamp plate is mounted to the booster piston, it is turned in the circumferential direction until it is engaged with the engagement pawls. However, when a valve sleeve is caused to rotate relative to the booster piston by turning operation of the clamp plate, the inner peripheral bead of the diaphragm may be twisted, resulting in the inner peripheral bead being disengaged from the opening edge of the valve sleeve fitting hole or the deterioration in durability of the diaphragm. To obviate the problems inherent to the conventional booster as mentioned above, it has been proposed that cutout portions are provided on the flange and are engaged with the outer peripheral surface of a fitting sleeve of a seal member for a tie rod in order to inhibit the valve sleeve from being rotated relative to the booster piston. However, this arrangement causes a structure for inhibiting the valve sleeve from being rotated to become complicated.
Furthermore, since the above-described conventional booster is so constructed that the outer peripheral part of the rear surface of the disengagement preventive flange is located very close to the fore surface of the booster piston, a fulcrum for forward flexing of the booster piston becomes located on the outer peripheral edge of the rear surface of the disengagement preventive flange. As a result, when the booster piston is flexed forwardly, this forward deflection is received by the aforesaid fulcrum. Accordingly, stress caused by the aforesaid flexing concentratively acts on a single place on the booster piston.